This is a truly stunning specimen of prasiolite (aka green quartz) from the amethyst mines near Artigas, Uruguay. It was mined from within a pocket in the basalt and features a secondary growth of sparkling green quartz crystals. Calcite crystals can be found scattered within this fascinating geode
I can pretty much guarantee this incredible piece will look even better in person than in the photos.
Prasiolite is a green variety of quartz that only naturally occurs in a few locations. It is formed when amethyst bearing rocks are heated by nearby lava flows or intrusions. It's often marketed under the name, green amethyst though that is technically incorrect as by definition amethyst is purple in color.